U.S. Pat. No. 7,713,111 discloses a control device for a water valve of a cut-off machine, which control device, during operation of a work tool, supplies a quantity of water, predetermined by the user, to the work tool. For this purpose, the work apparatus is connected with a water hose to a water supply, wherein the quantity of water supplied by the user is predetermined by an electronic setting unit. The signal of the setting unit is transmitted to a control device which as a function of the setting variable of the setting unit activates the drive coil of the electromagnetic valve with a pulse-width-modulated signal sequence. The quantity of water supplied by the electromagnetic valve can be set independently of the pressure of the water supply in such a manner that good dust binding at the cutting disk is ensured by the supplied quantity of water.
U.S. Pat. No. 7,126,449 discloses an electromagnetic valve for a fuel system, which valve, for the metering of a required quantity of fuel for a combustion engine, is acted upon by a control device with a pulse-width-modulated signal sequence. The pulse width of the operating pulses is determined by the control device as a function of operating parameters of the combustion engine and adapted over the operating duration of the combustion engine.
Irrespective of the use of an electromagnetic valve as a water valve, as a fuel valve or similar valve, the valve body of the valve may stick to the valve seat, may become jammed or may become sluggish in some other way. If an electromagnetic valve for setting a small volumetric flow is activated with only short operating pulses, a movement of the valve body may fail because of sticking, jamming or a similar obstruction. The valve body does not carry out any movement; the valve state remains unchanged despite energizing of the drive coil. It may thus happen that the valve does not open.